1. Field of the Invention
This invention relates to a heat recovering apparatus for a feeding engine, that is, a cogeneration system with an engine, adapted to recover the thermal energy of an exhaust gas as electric energy and hot water supplying energy by a heat exchanger.
2. Description of the Prior Art
In a diesel engine, light oil is generally burnt as a fuel in a combustion chamber. The light oil has a high cetane value and not so high a viscosity, and is suitable to disperse an atomized fuel uniformly into a combustion chamber. In order to greatly reduce the particulates and NOx components in an exhaust gas from a recent diesel engine, the air is compressed under a high pressure in a combustion chamber, and a fuel is injected into the resultant air, whereby the atomization of a gaseous mixture in the combustion chamber is attained. In general, the fuels used in an engine include gasoline, light oil, natural gas and heavy oil. A conventional generating set of a cogeneration system mostly comprises a system in which a generator is mounted on an engine. Since a diesel engine uses light oil as a fuel, the fuel cost increases in the generation of electricity by a diesel engine, so that an advantage concerning the generating cost cannot be secured.
A conventional cogeneration type engine using natural gas as a fuel is disclosed in, for example, Japanese Patent Laid-Open No. 33707/1994. This cogeneration type engine is adapted to generate steam by the exhaust gas energy, and the steam energy is recovered as electric energy, whereby a thermal efficiency is improved. In this engine, a turbocharger is driven by the exhaust gas energy, and a generator-carrying energy recovering apparatus by the exhaust gas energy from the turbocharger. The thermal energy of the exhaust gas from the energy recovering apparatus is converted into steam in a first heat exchanger, and a steam turbine is driven by this steam to recover the energy as electric energy, hot water being generated in a second heat exchanger by the high-temperature steam from the steam turbine to utilize the hot water as hot water to be supplied.
If heavy oil can be utilized as a fuel in a feeder using a diesel engine, the fuel cost can be reduced greatly. Therefore, in order to inject heavy oil at a high pressure into a combustion chamber, it is conceivable to give fluidity to the heavy oil by heating the same.
The applicant then developed diesel engines using heavy oil as a fuel, and filed patent applications therefor which included, for example, Japanese Patent Application Nos. 46956/1996, 151910/1996 and 151911/1996.
In a diesel engine, substantially 50% of the energy of a fuel is discharged in the form of exhaust gas. Therefore, when a diesel engine has been spread as a feeding engine throughout urban areas, the thermal energy discharged in the form of an exhaust gas poses problems. Namely, when the exhaust gas from an engine is discharged as it is to the atmospheric air, the ambient temperature increases to cause the environmental condition to be deteriorated. Moreover, the fuel cost increases, so that problems with saving of resources arise.
The recovering of the thermal energy of an exhaust gas discharged from an engine can be attained by providing a turbocharger and an energy recovering turbine in an exhaust pipe. For example, when an exhaust gas has 900.degree. C. thermal energy, a part of the energy is recovered by a turbocharger, and the temperature of the exhaust gas discharged from the turbocharger decreases to around 600.degree. C. The exhaust gas discharged from the turbocharger is passed through the energy recovering turbine, whereby the thermal energy is recovered to cause the temperature of the exhaust gas to decrease to around 500.degree. C. In order to further recover the thermal energy of the exhaust gas discharged from the energy recovering turbine, and reduce the temperature of the exhaust gas, a heat exchanger is provided on the downstream side of the energy recovering turbine.
In a cogeneration system with an engine comprising a diesel engine, it is necessary that a thermal efficiency be improved by using heavy oil or natural gas as a fuel and thereby reducing the fuel cost, and that the environmental condition be not deteriorated by the thermal energy discharged from the engine. Especially, in an urban area, it is necessary that the influence of the heat of the exhaust gas discharged from an engine be reduced. Therefore, a feeding engine has a problem of effectively recovering the thermal energy from an exhaust gas therefrom. To solve this problem, it is necessary that the construction of a heat exchanger, through which an exhaust gas and water are passed, be developed so as to enable the thermal energy of an exhaust gas to be recovered effectively, and excellent corrosion resistance, thermal resistance and durability to be displayed with respect to an exhaust gas and water.